1. Field of the Invention
The present invention relates to an organic light-emitting device that uses a bipolar compound in the emission layer to achieve improved efficiency and a long lifetime.
2. Discussion of the Background
Emission materials for organic electroluminescent devices are divided into fluorescent materials using a singlet exciton and phosphorescent materials using a triplet exciton according to the emission mechanism.
In general, phosphorescent materials are organic metal compounds containing heavy atoms. When phosphorescent materials are used, triplet-state excitons, which are forbidden transition, may produce phosphorescent emission. Phosphorescent materials have higher emission efficiency than fluorescent materials because phosphorescent materials use 75% triplet-state excitons while fluorescent materials use 25% singlet-state excitons.
An emission layer composed of a phosphorescent material includes a host material and a dopant material. The dopant material emits light by receiving energy from the host material. Examples of dopant materials include iridium metal compounds, such as (4,6-F2 ppy)2Irpic or an Ir compound based on a ppy(fluorinated ppy) ligand structure developed by Princeton University and South California University. CBP(4,4′-N,N′-dicarbazole-biphenyl) have been widely used as host material for these iridium compounds. A triplet state of CBP has an energy band gap that is sufficient for energy transition for green and red materials. However, CBP has a smaller energy band gap than blue materials which causes an ineffective heat-absorbing transition to occur. The energy transition from the CBP host to blue dopant is not sufficient, blue emission efficiency is low, and the lifetime of the device is short.